Much effort has been devoted to the development of light sources producing radiation in the so-called vacuum ultraviolet (vuv) region of the electromagnetic radiation spectrum, i.e., the region covering radiation which has a wavelength of between approximately 2 A and 2000 A and which is absorbed by any appreciable amount of gas. Reference is made to applicant's work, entitled Techniques of Vacuum Ultraviolet Spectroscopy, published by Wiley & Sons, N.Y. (1967), for an extensive discussion of the activity in this important research field.
Generally speaking, there are a number of features common to all vuv sources. With the exception of synchrotron radiation devices, all vuv sources operate on the principle of an electrical discharge in an ionizable gas or vapor. The electrical discharge is also typically confined to a capillary and is viewed "end-on". Since optically transparent materials are also highly absorbent to vuv radiation, vuv sources must be operated without windows. Several factors are known to affect the wavelength of the radiation emitted by a source, including the mechanism of discharge, the voltage employed to create the discharge, the amount of electrical current passed through the source, and the nature of the gas or vapor being ionized. Typically, a direct current (dc) discharge produces radiation of wavelengths longer than 900 A, whereas alternating current (ac) discharge tends to produce radiation of shorter wavelengths.
The nature of vuv radiation and its generation by gas discharge imposes stringent requirements on the design of vuv light sources, and the operating conditions of such sources place a severe strain on the apparatus. As a consequence, conventional gas discharge sources, an illustrative example of which is disclosed in U.S. Pat. No. 3,026,435 (McPherson), suffer from a number of disadvantages which have limited their utility.
A principal disadvantage of prior art sources is that the electrodes and capillaries are not readily replaceable without substantial dismantling of the apparatus, nor are electrodes and capillaries having differing dimensions and configurations easily substituted in such devices. As a consequence, conventional sources have only limited, if any, capability of operating as both a glow and spark discharge device. Further, the difficulty of modifying conventional sources and substituting components severely limits the ability to "fine tune", or optimize the performance characteristics of such sources for a variety of specialized applications.
Another major disadvantage of prior art gas discharge light sources is that although the desirability of minimizing the inductance of the electric circuit supplying power to the source for spark discharge operation thereof has been recognized, the sources themselves possess a relatively large inductance which limits the frequency and peak current at which such sources can be operated for a given capacitance. The inductance of the McPherson device, for example, is so large that ac operation thereof is limited to relatively low frequency, continuous wave currents.
A further disadvantage of conventional light sources is that the discharge capillary is water-cooled, such as, for example, by a "water-jacket" as disclosed in the McPherson patent, or by the arrangement shown in FIG. 5.62 of applicant's work Techniques in Vacuum Ultraviolet Spectroscopy, referred to hereinabove. Cooling of the capillary in this manner limits the size of the discharge current which can be attained because high current densities will wear away the capillary causing the water cooled capillary to break.
A still further disadvantage of the conventional light sources of the type exemplified by the McPherson patent is that a substantially uniform gas pressure is maintained throughout the source, including the capillary and front electrode, which results in a substantial reduction in the intensity of the light produced by such sources.